Technical Field
The present invention relates to methods and systems for removing selenocyanate (SeCN−) from contaminated aqueous media. More specifically, the present invention relates to methods and systems for removing selenocyanate and associated selenium species (selenite and selenate) from contaminated aqueous media by photocatalysis.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Selenium (Se) is a metalloid that is present in the earth's crust, often in associate with sulfur-containing minerals. It is normally found in low concentrations of 50-90 μg/kg, but higher concentrations can be associated with some volcanic, sedimentary and carbonate rocks. Selenium concentrations in soils vary widely, from 5 to 1,200,000 μg/kg, being higher in soils of more recent volcanic origin. Due to its multiple possible oxidation states, selenium occurs in soils in several forms: selenides (Se2−), amorphous or polymeric elemental selenium (Se0), selenites (Se4+) and selenates (Se6+).
Selenium also plays an important biological role in most, if not all animals, in the form of the amino acids selenocysteine and selenomethionine, as well as the proteins formate dehydrogenase and thioredoxin reductase. Selenium is therefore an essential micronutrient to many living organisms, but at high concentrations, selenium can pose serious health hazards. For example, accumulation of high concentrations of selenium in human can result in signs and symptoms such a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, neurological damage and, in extreme cases, cirrhosis of the liver, pulmonary edema, and even death.
Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability. By contrast, selenates and selenites that are largely generated by industries are very toxic, having an oxidant mode of action similar to that of arsenic trioxide.
High concentrations of selenium or selenium contamination are not limited to natural causes as previously set forth. Human activities that are major sources of selenium pollution include, for example, copper refinement, oil or petroleum refinement, mining, minerals processing, agriculture, and coal-based power generation.
More recently, selenocyanate has also generated attention as another highly toxic inorganic selenium-containing species that can be found in effluents from oil refineries and coal-based generating plants, including, in particular, refineries processing oil from selenium-containing stocks (e.g. seleniferous crudes, shales and coals) produced by High Pressure Injection (HPI) and clean coal plants using an integrated gasification combined cycle (IGCC) and flue gas desulfurization (FGD).
Selenium in the form of selenate and selenite can be removed from water by treating the water in biological reactors, for example as described in U.S. Pat. No. 6,183,644 and International Publication Number WO 2007/012181 (each incorporated herein by reference in its entirety), and as used in ABMet™ reactors sold by the General Electric Company. However, the ABMet™ system is not able to remove reduced selenium species such as selenocyanate and these systems require a large area of available land.
Selenocyanate may also be treated by precipitation using additives such as elemental iron or copper (II) salts. These processes consume the additives used to cause precipitation and create a large amount of hazardous sludge such as ferrous selenide and α-Cu(S0.91Se0.09)CN (U.S. Pat. Nos. 5,993,667, 6,214,238; Manceau, A., Gallup, D. L. Environ. Sci. Technol. 1997 31, 968-976; Meng, X, Bang, S., Korfiatis, G. P. Water Research 2002 36, 3867-3873—each incorporated herein by reference in its entirety).
It is noteworthy that while selenite is amenable to removal via conventional co-precipitation, selenate and selenocyanate are relatively recalcitrant towards most precipitation technologies.
Attempts have also been made to oxidize selenocyanate prior to introducing the adsorbent; oxidizing agents which have been employed have included air, ozone, hydrogen peroxide, and chlorine dioxide. None of these oxidants has proven entirely satisfactory: air is ineffective for oxidizing selenocyanate, while ozone and peroxide require highly alkaline conditions for maximum effectiveness. Chlorine dioxide oxidizes selenocyanate effectively at neutral pH, but it is unstable in aqueous solution and would have to be generated as needed.
U.S. Patent Application Publication 20130193071A1 describes a process of removing selenocyanate or selenite by contacting an aqueous effluent with a phenol oxidizing enzyme (i.e. peroxidase, laccase) and a suitable oxidizing agent such as hydrogen peroxide and oxygen (incorporated herein by reference in its entirety).
In view of the foregoing, there remains a need for solutions that can effectively remove different inorganic selenium-containing species from industrial effluent. Among the advantages that the present invention may offer include the ability to remove certain recalcitrant selenium species (e.g. selenocyanate) without generating by-products such as sludge and precipitate.